1. Field of the Invention
The present invention relates to amplifiers including a current mirror arrangement. The invention is particularly but not exclusively concerned with transconductance amplifiers in envelope tracking architectures.
2. Description of the Related Art
Frequency domain duplex (FDD) systems include transceivers that have a transmitter and a receiver which operate at different carrier frequencies. A simple exemplary architecture of such an FDD system is illustrated in FIG. 1. A transceiver includes a transmitter 6 which receives an input signal on line 2 to be transmitted by an antenna 18. A receiver 8 receives signals that are detected at the antenna and delivers them on signal line 4. The output of the transmitter 6 is delivered to a duplex filter 14 on line 10. A received signal detected at the antenna 18 is delivered from the duplex filter 14 on a line 12 to the receiver 8. The antenna 18 is connected to the duplex filter 14 via line 16.
In such an FDD system it is essential that energy from the transmitter does not block the receiver. This may occur because the duplex filter on the output of the transmitter has only limited attenuation. Any noise present at the receiver frequency on the transmitter output due to noise from the transmitter has the potential to cause receiver blocking.
The transmitter circuitry 6 typically includes a transconductance amplifier. A transconductance amplifier generates a current which is proportional to its input voltage. Any noise present at the receiver frequency on the transmitter output due to noise from a transconductance amplifier in the transmitter has the potential to cause the above-mentioned receiver blocking. This noise must therefore preferably not be allowed to exceed a low level while the power consumption of the transconductance amplifier is minimised. Thus efficient, low noise, high linearity transconductance amplifiers are required in FDD systems.
A typical transconductance amplifier has a high output impedance that sinks and/or sources current into a load that has as an input a very low impedance virtual earth. Such an arrangement is very efficient at ensuring a majority of the transconductance amplifiers current is fed into the load with minimal loss in the transconductance amplifier's output impedance.
However, a low impedance load reduces the power supply rejection ratio (PSRR) of transconductance amplifiers designed to operate from low voltage supply rails and requiring a large output dynamic range and low noise.
It is an aim of the invention to address one or more of the above-stated problems with an improved amplifier arrangement, such as an improved transconductance amplifier arrangement.